Nanoparticles are ubiquitous and by far the most abundant particle-like entities in natural environments on Earth and are widespread across many applications associated with human activities. There are many types of naturally occurring nanoparticles and man-made (engineered) nanoparticles. Nanoparticles occur in air, aquatic environments, rainwater, drinking water, bio-fluids, pharmaceuticals, drug delivery and therapeutic products, and in a broad range of many industrial products. Nanoparticles usually occur within polydisperse assemblages, which are characterized by co-occurrence of differently-sized particles
Given the widespread usage of nanoparticles, the ability to control and accurately characterize their properties may be useful to many applications. Conventional methods for measuring nanoparticle properties may be inaccurate for polydisperse samples of mixed nanoparticle sizes, which are common in many applications. Some of these conventional approaches make measurements on an ensemble of a large number of nanoparticles within a sample. Because the light scattered from all nanoparticles is measured simultaneously, it may be difficult to resolve the nanoparticles into their constituent sizes when there is a range of particle sizes. Other approaches fail to account for the large differences in the intensity of the scattered light produced by differently-sized nanoparticles across the range of nanoparticle sizes. In these approaches, the low scattering signals from small nanoparticles may be undetected, or the high scattering signals from larger nanoparticles can obscure the signals from smaller nanoparticles. As a result of these deficiencies, the concentration of nanoparticles of any given size, and hence the entire size distribution, can be subject to unknown error.
These methods of detecting nanoparticles are commonly referred to as dark field microscopy. The instrument to perform such an analysis typically comprises a small cell (for example, a cuvette) that enables illumination of a liquid with a precisely defined, narrow light sheet and observation of scattered light from the nanoparticles, usually at a 90-degree angle relative to the light sheet plane. In other words, the direction of observation is perpendicular to the direction of the plane of illumination. Different sizes of particles can be visualized via the digital camera capturing light scattered by particles, with images having different sizes and intensities (various brightness of pixels) depending on the size of the particles. But as noted above, recording images of light scattered by particles of mixed sizes coexisting in a solution remains somewhat problematic, due to the huge difference in the amount of light scattered by particles of different sizes. Specifically, the intensity of scattered light depends very strongly on the particle size, changing by many orders of magnitude between 10 nm and 1000 nm diameter nanoparticles, for instance. This problem is also encountered in other areas of photography and videography and is commonly called High Dynamic Range (HDR) imaging. What has been needed is an improved system and method that overcomes the problems that tend to be inherent in dark field microscopy systems.
It is well known that the intensity of scattering of light with wavelengths comparable to the size of scattering particles (for visible light wavelengths that are between 400 nm and 700 nm) depends in a complicated way on the particle's size. See Mie scattering, G. Mie 1908 Annalen der Physik 330(3) pp. 377-445. The intensity varies vastly by orders of magnitude when the size of particles changes just by a factor of 10: for particles smaller than about 100 nm, this dependence is proportional to d6; that is, scattering cross-section drops as the sixth power of particle's diameter. When one deals with the highly polydisperse colloid that contains particles of many different sizes, visualization of all such particles is problematic when one uses a simple monochromatic light source with a fixed intensity. Either small particles are not visible when the large ones are scattering not too much light to not blind (oversaturate) the camera, or, when one uses a much stronger light source that enables the visualization of small particles, images of large ones are highly overexposed and typically have very irregular contours due to interference that are hard to track in time (finding a center of such irregular shapes is very complicated and time consuming, not practical for computerized processing).
To solve the problem of wildly varying scattered light intensities, in the past patent applications the inventor used multiple lasers with different colors of beams and different intensities of emitted light. The images were recorded by a color video camera, if all the lasers were working simultaneously (U.S. Pat. No. 9,645,070), or by greyscale camera, if the lasers were triggered sequentially (U.S. Pat. No. 10,161,852).
But even these systems could overly excite the particles under investigation, leading to misestimating the particle size. For example, in FIG. 1 of U.S. Pat. No. 9,541,490, an apparatus is shown with one laser and a cylindrical lens with a focal length of 50 mm that squeezes a light beam emerging from a laser aperture (typically 2 mm×2 mm or circular shape with diameter of about 1 mm). An additional objective with a power of 5× makes a laser sheet inside the cuvette with a thickness of about 50 microns. If the applied laser has a maximal power of 1 W at aperture, then taking into account losses in the above-mentioned optical system, one obtains about 400 mW of light power inside the cuvette, with the cross-section of the laser sheet having dimensions of the already-mentioned 50 microns by about 400 microns. This results in a laser light intensity of about 2 kW/cm2, which is a considerable power density. For comparison, the so-called solar constant, or intensity of solar illumination on the surface of Earth, is about 0.136 W/cm2 or about 15,000 times less intensity than in the described laser sheet. A special microscope with a long working distance (LWD) objective allows for the observation of light sheet deep inside the cuvette, mitigating somewhat the thermal eddies formed by the intense energy from the laser. Indeed, that is why the '490 patent discloses a special insert to mitigate the thermal convection of the colloid due to evaporation on the surface.
What is needed, therefore, is an improved system that overcomes these problems and allows for fast intensity changes that can be synchronized to a sensor's frame rate. Ideally, the system should be simple and easy to manufacture, with very few elements forming changeable light sheets, and thus lowering the cost of manufacturing, and making use and service simple.